For the uses of photopolymer formulations, the crucial role is played by the refractive index contrast Δn produced in the photopolymer by the holographic exposure. In holographic exposure, the interference field of signal light beam and reference light beam (that of two planar waves in the simplest case) is mapped into a refractive index grating by the local photopolymerization of, for example, high-refractive acrylates at loci of high intensity in the interference field. The refractive index grating in the photopolymer (the hologram) contains all the information of the signal light beam. By illuminating the hologram with only the reference light beam, the signal can then be reconstructed. The strength of the signal thus reconstructed relative to the strength of the incident reference light is called the diffraction efficiency, DE in what follows.
In the simplest case of a hologram resulting from the superposition of two plane waves, the DE is the ratio of the intensity of the light diffracted on reconstruction to the sum total of the intensities of incident reference light and diffracted light. The higher the DE, the greater the efficiency of a hologram with regard to the amount of reference light needed to visualize the signal with a fixed brightness.
High-refractive acrylates are capable of producing refractive index gratings with high amplitude between regions with low refractive index and regions with high refractive index, and hence of enabling holograms with high DE and high Δn in photopolymer formulations. It should be noted here that DE depends on the product of Δn and the photopolymer layer thickness d. The breadth of the angle range at which the hologram is visibly (reconstructed), for example under monochromatic illumination, depends solely on the layer thickness d.
On illumination of the hologram with white light, for example, the breadth of the spectral range which can contribute to the reconstruction of the hologram likewise depends solely on the layer thickness d. The smaller d is, the greater the respective breadths of acceptance. Therefore, if the intention is to produce bright and readily visible holograms, the aim is a high Δn and a low thickness d, so as to maximize DE. This means that, the higher the Δn, the more freedom is achieved to configure the layer thickness d for bright holograms without loss of DE. Therefore, the optimization of Δn is of major importance in the optimization of photopolymer formulations (P. Hariharan, Optical Holography, 2nd Edition, Cambridge University Press, 1996).
WO 2008/125229 discloses photopolymer formulations comprising mono- and difunctional writing monomers of high molecular weight. Media made from these formulations can be used to write reflection holograms of good suitability for data storage, for example. However, problems occur in the production and processing of the formulations: For instance, the writing monomers present have a high viscosity or high TG values (TG=glass transition temperature). This means that it is difficult to achieve homogeneous distribution of the writing monomers in the photopolymer formulation and a medium produced therefrom. Moreover, when the known formulations are used, writing monomer agglomerates can be formed in the polymer matrix, which considerably impairs the quality of the media or the holograms exposed therein. In such cases, the holographic materials become hazy.
A particular form of holograms is that of transmission holograms, a feature of which is that, in the course of production of the holograms, the reference beam and the object beam irradiate the holographic medium from the same side. Transmission holograms find various uses. Particular mention should be made here of the light guide as diffractive optical element. Such an optical element can be used in demanding applications such as spectroscopy or astronomy. They are likewise suitable for use in electronic displays, for example in 3D displays.
Because of the geometry of the interfering object and signal beams, the lattice spacing in transmission holograms is large compared to reflection holograms. According to the wavelength, it may be between 500-1000 nm. Since the mechanism of hologram formation in the photopolymer is based on the diffusion of the writing monomers, there is a need for writing monomers which can diffuse far enough with the large lattice spacing customary for transmission holograms. However, this is a prerequisite for being able to enable a high refractive index contrast (Δn). The photopolymers known from the field of reflection holograms are frequently unsuitable for this purpose, or do not lead to a sufficiently high refractive index contrast.
WO 2012/020061 describes specific writing monomers containing (meth)acrylate groups and based on thioethers, and also photopolymer formulations and media comprising them, especially for recording transmission holograms. However, the process used to prepare the writing monomers has disadvantages: Thus, in a first step, epoxides and thiols are reacted under catalysis to give hydroxy-functional thioethers, which are then reacted in a second step with isocyanate-functional acrylates. However, the reaction of the thiols is a comparatively slow reaction, and full conversion of the usually highly odorous reactants is difficult to achieve. The handling of these materials and the cleaning of reaction vessels in the production environment also entail a relatively high level of cost and inconvenience. Moreover, unconverted free thiols inhibit free-radical polymerization, and so it would be desirable to identify alternatives to the products described that can be prepared in a thiol-free manner.
Moreover, a disadvantage of the writing monomers of WO 2012/020061 and of WO 2008/125199 is that they have high viscosity and are pasty materials. This makes it difficult to produce photopolymer formulations, since efficient, rapid, homogeneous mixing of the components is achievable. Moreover, these high-viscosity writing monomers are difficult to filter and generally disadvantageous to handle. Moreover, it may also be necessary to use organic solvents, which is disadvantageous for reasons of occupational safety and environmental protection. Furthermore, it is disadvantageous to use solvents in production of holographic media, especially with high layer thicknesses, since they can be removed, for example, from media in the form of films only with a relatively high level of complexity, in which case, however, there can again be impairment of the quality, for instance in the form of surface defects. However, such defects are unacceptable in many cases, since the media can then no longer achieve the high-precision optical functions for which they are intended.
Moreover, it is generally advantageous in the development of photopolymer formulations when the writing monomers have a high solubility in the further components. Thus, in this case, the quantitative ratios of the components in the formulation can be varied within a relatively wide ranges, which considerably eases adaptation to specific applications or actually makes it possible at all.